code: def stack(*fields: str) -> Expr:
      """Create a Create a ``DataSpec`` dict to generate a ``Stack`` expression
      for a ``ColumnDataSource``.
  
      Examples:
  
          .. code-block:: python
  
              p.vbar(bottom=stack("sales", "marketing"), ...
  
          will generate a ``Stack`` that sums the ``"sales"`` and ``"marketing"``
          columns of a data source, and use those values as the ``top``
          coordinate for a ``VBar``.
  
      """
  
      return Expr(Stack(fields=fields))
  
file_path: bokeh/src/bokeh/transform.py
imports: from __future__ import annotations
  import logging
  from typing import TYPE_CHECKING, Sequence, Union
  from .core.property.vectorization import Expr, Field
  from .models.expressions import CumSum, Stack
  from .models.mappers import CategoricalColorMapper, CategoricalMarkerMapper, CategoricalPatternMapper, EqHistColorMapper, LinearColorMapper, LogColorMapper
  from .models.transforms import Dodge, Jitter
label_desc: Use image node_id 10 for calling a global function with example usage: stack() and returns: Expr